Unraveling the Structural Evolution of Cobalt Sulfides in Electrocatalytic NO3RR: the Inescapable Influence of Cl–

Electrochemical nitrate reduction (NO₃RR) to ammonia is an attractive approach for mitigating NO₃^– pollution and producing valuable NH₃. Cobalt–sulfur compounds are widely considered to be potential electrocatalysts for NO₃RR. However, there is still a lack of research on the probable structural evolution, long-term stability, and reactive sites of cobalt-based sulfides during catalysis. Herein, we have employed three cobalt sulfides (CoS_x, where x = 8/9, 2, 1.097) with different sulfur contents as catalysts for electrocatalytic NO₃RR under alkaline conditions. At −0.8 V vs RHE, all these CoS_x show promising performances that Faradaic efficiencies of >80% and a high yield of >1780 mmol h^–1 g_cat^–1 for NH₃ production are achieved. Through a combination of X-ray diffraction (XRD), transmission electron microscopy (TEM), and other characterizations, it is revealed that all these cobalt sulfides are easily converted into cobalt hydroxide during the NO₃RR. This phenomenon is seemingly contradictory to the thermodynamic prediction that, according to the Pourbaix diagram, these CoS_x compounds should be stable even under the catalytic condition. We suggest that this is due to the presence of Cl^– ions in the electrolyte that promote the transformation of CoS_x toward Co(OH)₂. Chloride ions are commonly found in both industrial settings and natural water bodies and are challenging to remove. The evolved Co(OH)₂ species is proposed to be responsible for catalyzing NO₃RR, especially during a long-term catalytic process. This study highlights the inevitable structural evolution of CoS_x catalysts under current alkaline electrocatalytic NO₃RR conditions, offering theoretical guidance for the judicious selection and design of future catalysts.